Sorption-desorption of dicamba and 3,6-dichlorosalicylic acid in soils

Sorption-desorption of the herbicide dicamba (3,6-dichloro-2-methoxybenzoic acid) and its metabolite 3,6-dichlorosalicylic acid (DCSA) was studied in soils using experimental conditions that modeled selected physical and chemical properties of soils. Changing the soil/solution ratio (1:1 and 1:10) and temperature (15,25, and 35 degrees C) of the soil slurry had no significant effect on sorption of dicamba for the soils studied, except for the Elkton (Typic Ochraquults) Ap where, at a 1:1 soil/solution ratio, Freundlich constants, Kf, of 0.8 and 0.07 were found at 15 and 25 degrees C, respectively. The DCSA sorbed to all five soils under all experimental conditions with Kf values ranging from 1.23 for the Evesboro soil (Typic Quartzipsamments) to 34.04 for the Hagerstown Bt soil (Typic Hapludalfs). Changing the soil/solution ratio from 1:1 to 1:10 did not significantly change the Kf, except for the Hagerstown Bt, where an increase in Kf value from 27.54 to 29.79 was obtained. Increasing temperature from 15 to 35 degrees C generally resulted in a decreased Kf value for DCSA. Both dicamba and DCSA showed increased sorption with decreasing equilibrium solution pH. The percent of sorbed DCSA that was desorbed after two washings with deionized water was 28.2% or less for these soils. Dicamba was readily desorbed (79.2%) after two washings from the Elkton soil. Dicamba showed increased sorption when the Hagerstown Ap was saturated with Fe3+, but did not show any response to Cu2+, Al3+, and Ca2+ saturations. The DCSA showed increased sorption to all four cation saturations. A complexation bonding mechanism between DCSA and polyvalent surface cations is proposed. For limed agricultural soils, dicamba would be highly mobile in both topsoil and subsoil matrices. Once dicamba degrades to DCSA, however, sorption can be expected to become significant. Under field conditions, DCSA sorption can be expected to increase with soil depth, especially where soil pH temperature decrease and exposed surface metal groups increase.